A set of sentences transmitted by some Garmin units in response to a received $PGRMO,GPALM,1 sentence. It can also be received by some GPS units (eg. Garmin GPS 16 and GPS 17) to initialize the stored almanac information in the unit.

This sentence is sent by some GPS receivers to allow them to be used to control an autopilot unit. This sentence is commonly used by autopilots and contains navigation receiver warning flag status, cross-track-error, waypoint arrival status, initial bearing from origin waypoint to the destination, continuous bearing from present position to destination and recommended heading-to-steer to destination waypoint for the active navigation leg of the journey.

Note: Some autopilots, Robertson in particular, misinterpret "bearing from origin to destination" as "bearing from present position to destination". This is likely due to the difference between the APB sentence and the APA sentence. for the APA sentence this would be the correct thing to do for the data in the same field. APA only differs from APB in this one field and APA leaves off the last two fields where this distinction is clearly spelled out. This will result in poor performance if the boat is sufficiently off-course that the two bearings are different.

Example 1: $GPBOD,099.3,T,105.6,M,POINTB,*01Waypoint ID: "POINTB" Bearing 99.3 True, 105.6 Magnetic This sentence is transmitted in the GOTO mode, without an active route on your GPS. WARNING: this is the bearing from the moment you press enter in the GOTO page to the destination waypoint and is NOT updated dynamically! To update the information, (current bearing to waypoint), you will have to press enter in the GOTO page again.

Example 2: $GPBOD,097.0,T,103.2,M,POINTB,POINTA*52This sentence is transmitted when a route is active. It contains the active leg information: origin waypoint "POINTA" and destination waypoint "POINTB", bearing between the two points 97.0 True, 103.2 Magnetic. It does NOT display the bearing from current location to destination waypoint! WARNING Again this information does not change until you are on the next leg of the route. (The bearing from POINTA to POINTB does not change during the time you are on this leg.)

eg. $GPGSV,3,1,11,03,03,111,00,04,15,270,00,06,01,010,00,13,06,292,00*74 $GPGSV,3,2,11,14,25,170,00,16,57,208,39,18,67,296,40,19,40,246,00*74 $GPGSV,3,3,11,22,42,067,42,24,14,311,43,27,05,244,00,,,,*4D $GPGSV,1,1,13,02,02,213,,03,-3,000,,11,00,121,,14,13,172,05*62 1 = Total number of messages of this type in this cycle 2 = Message number 3 = Total number of SVs in view 4 = SV PRN number 5 = Elevation in degrees, 90 maximum 6 = Azimuth, degrees from true north, 000 to 359 7 = SNR, 00-99 dB (null when not tracking) 8-11 = Information about second SV, same as field 4-7 12-15= Information about third SV, same as field 4-7 16-19= Information about fourth SV, same as field 4-7

This sentence is transmitted before each individual sentence where there is a need for the Listener to determine the exact source of data in the system. Examples might include dual-frequency depthsounding equipment or equipment that integrates data from a number of sources and produces a single output.

eg1. $GPWPL,4917.16,N,12310.64,W,003*65 4917.16,N Latitude of waypoint 12310.64,W Longitude of waypoint 003 Waypoint ID When a route is active, this sentence is sent once for each waypoint in the route, in sequence. When all waypoints have been reported, GPR00 is sent in the next data set. In any group of sentences, only one WPL sentence, or an R00 sentence, will be sent. eg2. $GPWPL,5128.62,N,00027.58,W,EGLL*59 1 2 3 4 5 6 1 5128.62 Latitude of nth waypoint on list 2 N North/South 3 00027.58 Longitude of nth waypoint 4 W East/West 5 EGLL Ident of nth waypoint 6 *59 checksum

This sentence is used on Garmin eTrex summit, Vista and GPS76S receivers to output the value of the internal flux-gate compass. Only the magnetic heading and magnetic variation is shown in the message.

Format of latitudes and longitudes

Where a numeric latitude or longitude is given, the two digits immediately to the left of the decimal point are whole minutes, to the right are decimals of minutes, and the remaining digits to the left of the whole minutes are whole degrees.

eg. 4533.35 is 45 degrees and 33.35 minutes. ".35" of a minute is exactly 21 seconds.

eg. 16708.033 is 167 degrees and 8.033 minutes. ".033" of a minute is about 2 seconds.

References

This information on NMEA sentences has been sourced from all over the 'net and I make no apologies for any inaccuracies or errors. Still, it's useful stuff. I wish to thank all the sources, which are listed on my GPS Links page. Please contact me if you know of freely available interpretations of sentences which are not on this page.